In the packaging of device dies that comprise integrated circuits, device dies may be molded in a molding compound at wafer level, and redistribution lines are formed to connect to the device dies. To mold the device dies in the molding compound, the device dies are first mounted on a glass wafer. The glass wafer, the device dies, and the molding compound in combination are handled as a composite wafer, and are transported. During the process steps performed on the composite wafer, the composite wafer is secured on chuck table the same way as a normal semiconductor wafer.
Due to the significant difference between the Coefficient of Thermal Expansion (CTE) of the molding compound and the CTE of the glass wafer, the composite wafer has warpage. The warpage may be as high as about 1.5 mm or higher. This posts a problem for the tool that picks up the composite wafer and transports it. For example, a suction head may be used to pick up the composite wafer and place it on a chuck table. Due to the warpage, it is difficult to form vacuum between the suction head and the composite wafer, which vacuum is used to attach the composite wafer to the suction head.